Silicon sheet material or ribbon is particularly important in making low cost solar cells. Continuous growth of silicon ribbon obviates the need for slicing of bulk produced silicon. Methods for doing this are described in U.S. Pat. Nos. 4,594,229; 4,627,887; 4,661,200; 4,689,109; 6,090,199; 6,200,383; and 6,217,649, the disclosures of which are herein incorporated by reference in their entireties. In these patents, continuous silicon ribbon growth is carried out by introducing two high temperature material strings up through a crucible that includes a shallow layer of molten silicon. The strings serve to stabilize the edges of the growing ribbon and the molten silicon freezes into a solid ribbon just above the molten layer. The molten layer that forms between the strings and the growing ribbon is defined by the meniscus of the molten silicon. U.S. Pat. Nos. 6,090,199 and 6,217,649 describe a method and apparatus for continuous replenishment of the feedstock material in a continuous silicon ribbon.
In order to produce lower cost solar cells and hence expand large-scale electrical applications of solar electricity, it is important to have lower cost and higher quality substrate materials for making the solar cell. The current invention provides new and improved methods and apparatus for growing silicon ribbons.